4. Cannabis the drug

4.8 Detection of cannabinoids in body fluids

Cannabinoid levels in the body, which depend on both the dose given and the smoking history of the individual, are subject to substantial individual variability. Plasma levels of THC in man may range between 0-500ng/ml, depending on the potency of the cannabis ingested and the time since smoking. For example, blood levels of THC may decline to 2ng/ml one hour after smoking a low potency cannabis cigarette, a level that may be achieved only nine hours after smoking a high potency cannabis cigarette. In habitual and chronic users such levels may persist for several days after use because of the slow release of accumulated THC.

The detection of THC in blood above 10-15ng/ml provides presumptive evidence of "recent" consumption of cannabis, but it is not possible to determine how recently it was consumed. A somewhat more precise estimate of the time of consumption may be obtained from the ratio of THC to 9-carboxy-THC: similar concentrations of each in blood could be an indication of use within the last 20-40 minutes, and would predict a high probability of the user being intoxicated. When the levels of 9-carboxy-THC are substantially higher than those of THC, ingestion can be estimated to have occurred m more than half an hour ago (Hawks, 1982; Perez-Reyes et al, 1982). However, such an interpretation probably applies only to the naive users who have resting levels of zero. Background levels of cannabinoids (particularly 9-carboxy-THC) in habitual users make the estimation of time of ingestion almost impossible. It is very difficult to determine the time of administration from blood concentrations of THC and its metabolites, even if the smoking habits of the individual and the exact dose consumed are known. The results of blood analyses indicate, at best, the "recent" use of cannabis.

Urinary cannabinoid levels provide an even weaker indicator of current cannabis intake. In general, the greater the level of cannabinoid metabolites in urine, the greater the possibility of recent use, but it is impossible to be precise about how "recent" use has been (Hawks, 1982). Only minute traces of THC itself appear in the urine due to its extensive metabolism, and most of the administered dose is excreted in the form of metabolites in faeces and urine (Hunt and Jones, 1980). 9-carboxy-THC is detectable in urine within 30 minutes of smoking. This and other metabolites may be present for several days in first time or irregular cannabis users, while frequent users may continue to excrete metabolites for weeks or months after last use because of the accumulation and slow elimination of these compounds (Dackis et al, 1982; Ellis et al, 1985). As with blood levels, there is substantial human variability in the metabolism of THC, and no simple relationship between urinary levels of THC metabolites and time of consumption. Hence, urinalyses results cannot be used to distinguish between use within the last 24 hours and use more than a month ago.
Several studies have examined measures of cannabinoids in fat and saliva. Analyses of human fat biopsies confirm storage of the drug for at least 28 days (Johansson, et al, 1987). Detection of cannabinoids in saliva holds more promise for forensic purposes, since it has the capacity to reduce the time frame of "recent" use from days and weeks to hours (Hawks, 1982; Gross et al 1985; Thompson and Cone, 1987).